Connecting speaker signals to line level inputs

Copyright Tomi Engdahl 1997

Sometimes it is necessary to connect a device which has only speaker
outputs to external amplifier's line level inputs. Because speaker
levels are much higher than line level signals, the direct connection
does not usually give satisfactory results. That's the reason why I developed
this signal level attenuator for connecting speaker signals to line
level inputs without signal level problems.

The circuit

The circuit is designed to work with typical HIFI amplifiers where
negative speaker connector (black) is connected to amplifier ground.
Be careful here because some modern amps have both terminals actively
driven. Connecting either one to a common reference could cause the
output stage to smoke.

Technical specs

The circuit is suitable to be used with 1..50W audio amplifers.
The circuit is connected to the speaker output of the amplifier and
can be used in parallel with the existing speakers.

Description of the circuit

Speaker level signals are usually in 3..20V range (amplifiers up to 50W output
power). The line level signals should be in 0.3..2V range to be suitable for
amplifier's line level input. So attenuation of about 20 dB is needed to
bring the signals to right level. This conversion can be done easily using
10:1 voltage divider circuit.

The circuit below is a resistor voltage divider circuit which just does
about 10:1 voltage attenuation. Input impedance of the circuit is
about 10 kohm, so it does not load the amplifier driving it in any
significant way and can be used in parallel with existing speakers
without any problems. The output impedance of the circuit is about
1 kohm, which is well suitable for normal line inputs.

Component list:

R1 10 kohm resistor 0.25W
R2 1 kohm resistor 0.25W

The resistor power ratings can be this low because this
circuit will only take a very small fraction of the power
that the amplifier tires to put out.
You can use this circuit as such with an amplfier or
you can connect it in parallel with existing speakers as well.

Notes on using the circuit

Remember to check that the circuit is connected in the right way
every time you use it. If you mix up speaker out and speaker ground
connection, you can end up shorting speaker output to ground, which
can damage your amplifier and/or other equipment connected to it.
If you want to reduce the risk of damaging anythign then
put 10 ohm resistor between the circuit and speaker ground connector
(this will make sure that short circuit impedance is always at least
10 ohms, though it can introduce sometime hummign problems because of
increase grounding impendace).

Remember that this circuit is designed only
to be used with normal amplifiers which have common ground for speaker
signals. It is not suitable to be used with bridge type output car
stereo amplifiers.

If you are planning to use thie circuit with amplifier where there
are no spakers connected to them some amplifiers might have
poower sound quality if there is nothign connected to speaker output.
If you experience this kind of problem, then you can put a 8 ohm resistor
(or whatever the apaker impedance that amplifier normally runs is) and
use it as a load. Remeber to select a resistor which cna handle the
power that the amplifier output. If you want to get around using a
lower power resistor, then usually a resistor in 50-300 ohm range gives
satisfactory results (instead of nominal 8 ohm). Anyway in most
cases you don't need this kind of loading resistors.

Ideas for modifications

Adjustable output level

If you want to adjust the output level then replace R2
with 1 kohm trimmer. This makes it possible to make the
output level to be adjustable from 0V to line level.

Using the circuit with more powerful amplifiers

If you want to use the circuit safely with more powerful
amplifiers than 50W, then you have to modufy the circuit
to give more attenuation. This can be easily done by
changing the value of R1 to 33 kohm (0.25W power rating).
This makes the attenuation to be 33:1 (around 30 dB). With this attenuation
the circuit is usable to amplifiers up to about 300W.
Remember to construct the circuit very carefully if you are going
to use it with those high power amplifiers because the voltage
on their speaker lines are at dangerous levels at high sound levels.

Other related circuits

Simple resistor attenuator is very useful also in many other
applications than just the speaker level to line level converters.

+4 dBm pro level to instrument amplifier input attenuator

HiFi video audio input attenuator

Modern Hifi-VHS VCRs don't typically have automatic level adjustment
or manula adjustment of recording levels. The dynamics of the HiFi-VHS
video soundtrack is so large that the adjustments are quite often
unnecessary. But in some cases some satellite boxes output higher than
the pecs say signals and if they are connected to HiFi-VHS video the
sound can distort in largest signals. To avoid this you can use the
following small attenuator in the HiFi VCR audio connections
(you need one for left audio channel and one for right channel
and you can built the circuit inside SCART EURO-AV connector if
you equipments are connected together using them).